Continuous Positive Airways Pressure (CPAP) breathing therapy machines are well known in the art for use in the treatment of sleep disordered breathing by supplying a continuous positive pressure to a patient's airway while the patient sleeps. A typical CPAP apparatus is programmed with a therapy pressure, and is able to maintain the set pressure (measured either at the mask or at a base unit) during the inhalation and exhalation phases of the breathing cycle. The pressure setting is typically programmed via a control on the unit. Bi-PAP machines will typically vary the positive pressure delivered to the user during the inhalation and exhalation phases of the breathing cycle. Typically, Bi-PAP machines deliver a lower pressure during the exhalation phase of the breathing cycle, to make it easier or less uncomfortable for patients to exhale while using the machine. The Bi-PAP machine is typically programmed with a therapy pressure, which is used as the inhalation pressure, while the exhalation pressure is typically a standard difference from the inhalation pressure.
FIG. 1 shows a schematic view of a typical prior art CPAP/Bi-PAP machine 30. Positive pressure is maintained by regulated blower 40, under control of motor control circuitry 38. Blower 40 supplies a pressurized flow of air to a mask connected via the flexible tube (not shown) to blower 40. Regardless of whether the device is a CPAP machine or a Bi-PAP machine, microprocessor 34, in accordance with normal operating programming stored in memory 36 produces a motor control signal which is interpreted by motor control circuitry 38. Motor control circuitry 38 translates motor control signal 37 into electrical impulses that control the speed of blower 40 to produce the desired pressure through flow element 42 and ultimately to the user of the device. The machine may be equipped with various sensors, such as pressure sensor 44 and flow sensor 46 to aid in the detection of sleep events. Control and programming of the device is accomplished via user interface 32.
The term “inspiratory flow limitation” describes a physiological condition in which the respiratory pattern is defined by constant or decreasing air flow without pressure dependence throughout significant portions of inspiration. This flow limitation is commonly caused by a narrowing of the upper airway. The pattern can be identified by a flattening of areas of the inspiratory waveform, as detected in the CPAP patient interface, resulting in an inspiratory waveform contour having plateaus which correlate to an elevated upper airway resistance.
The ability to detect the flow limitation condition is important for several reasons. If the increased airway resistance is sufficiently high, the tidal volume will fall. If the high upper airway resistance and reduced tidal volume persist, sleep-disordered breathing events will likely occur in the form of hypopneas or even apneas. Furthermore, if sufficient inspiratory muscle effort is required to overcome the flow limitation, transient arousals from sleep may occur, which may lead to daytime somnolence. Titration studies have also shown that flow limitation may curtail periods of deep sleep, even when the patient is not aroused.
In the context of a CPAP breathing therapy device, the ability to reliably detect a flow limitation condition can be used as an aid in adjusting the therapy pressure. Typically, a breathing therapy machine will seek to deliver the lowest possible therapeutically effective pressure to the patient, such as to minimize any patient discomfort associated with the use of the device. Typically, such devices will slowly lower the therapy pressure until the patient experiences an event, then will raise the pressure to stop the events, and then will return to slowly lowering the pressure. The ability to detect flow limitations as the pressure is lowered can act as a warning mechanism that further lowering the pressure may precipitate the occurrence of events. Therefore, when the flow limitation condition is detected, the CPAP machine can stop the lowering of the pressure, or increase the pressure to alleviate the flow limitation condition.
There are many methods of detecting flow limitation extant in the literature and several have been implemented in prior art CPAP machines. However, it has been found that prior art methods are unreliable and may often miss detecting flow limitation conditions, particularly in situations where the airflow waveform does not exhibit well-defined plateau areas in the inspiratory portion of the waveform. Therefore it would be desirable to have a more reliable method of determining the flow limitation condition.